Solar power generation with multiple energy conversion modes

ABSTRACT

A multi-mode solar power generation system can include a first energy conversion system that generates electricity from a working fluid heated by a portion of solar radiation focused by a plurality of heliostats. The multi-mode solar power generation system can also include a second energy conversion system that generates electricity from an unused portion of the focused solar radiation using a different energy conversion mode than that of the first energy conversion system. The second energy conversion system can include one or more photovoltaic converters, which directly convert solar radiation to electricity. The unused radiation from the first energy conversion system can include radiation spillage or dumped radiation from a thermal receiver of the first energy conversion system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 61/036,959, filed Mar. 16, 2008, U.S. ProvisionalApplication No. 61/053,341, filed May 15, 2008, and 61/140,966, filedDec. 28, 2008, all of which are hereby incorporated by reference hereinin their entireties.

FIELD

The present disclosure relates generally to the conversion of solarradiation to electric power, and, more particularly, to the conversionof concentrated solar radiation to electric power using multiple energyconversion modes.

SUMMARY

A solar power generation system can have a thermal-electric powergeneration component, in which incident solar radiation is concentratedon a thermal receiver to heat a heat transfer or working fluid for usein electricity generation. A field of heliostat-mounted mirrors canreflect and concentrate incident solar radiation onto the thermalreceiver. A portion of the reflected solar radiation may be unused ordumped from the thermal receiver due to one or more operatingconditions. Reflected solar radiation that would otherwise be dumped canbe captured by an alternate receiver or receiver portion using one ormore supplemental energy conversion components. For example,concentrated sunlight dumped or spilled onto a photovoltaic convertercan be directly converted to electricity. Such a photovoltaic convertermay be located near or adjacent the thermal receiver of the thermalelectric power generation component to minimize any displacement ofheliostats that may be necessary for dumping purposes. The outputderived from the supplemental conversion components may be captured andcombined with existing systems employed in the thermal-electricgeneration of power.

Objects and advantages of the present disclosure will become apparentfrom the following detailed description when considered in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Where appropriate, like reference numbers have been used to indicatelike elements in the figures. Unless otherwise noted, the figures havenot been drawn to scale.

FIGS. 1 and 2 are diagrammatic elevation views of a plurality ofheliostats and a central power tower in accordance with differentembodiments of the invention.

FIGS. 3A and 3B are plan and elevation views, respectively, of astar-shaped receiver with an array of photovoltaic elements.

FIGS. 3C and 3D are plan and elevation views, respectively, of acylindrical receiver with two arrays of photovoltaic elements.

FIGS. 3E and 3F are plan and elevation views, respectively, of a pair ofpolygonal receiver portions with an array of photovoltaic elements.

FIG. 3G is an elevation view of a receiver with a repositionable arrayof photovoltaic elements.

FIGS. 3H-3K are cross-sectional views of different positions of an arrayof photovoltaic elements.

FIGS. 4A and 4B are plan and elevation views, respectively, of areceiver with an array of photovoltaic elements and a cooling system.

FIG. 4C is an elevation view of a receiver with an array of photovoltaicelements and a fluid preheater.

FIGS. 5A, 5B, 6A, and 6B are diagrammatic elevation views of a heliostatand a plurality of power towers illustrating the incidence andreflection angles of solar radiation with respect to focusingalternately on each of the towers.

FIGS. 7A and 7B are diagrammatic elevation views of heliostats and powertowers illustrating the alleviation of shadowing with respect tofocusing alternately on each of the towers.

FIGS. 8A and 8B are diagrammatic elevation views of heliostats and powertowers illustrating the alleviation of blocking with respect to focusingalternately on each of the towers.

FIGS. 9A and 9B are diagrammatic elevation views of heliostats and powertowers illustrating the implementation of a secondary reflector.

FIG. 10 is an isometric view of a solar receiver with a repositionablesecondary reflector.

FIGS. 11A and 11B are plan and isometric views, respectively, of asquare receiver with photovoltaic-element-containing projections.

FIG. 12 is a plan view of a square receiver with an alternatearrangement for photovoltaic-element-containing projections.

FIGS. 13A and 13B are isometric and elevation views, respectively, of areceiver with a pair of photovoltaic-element-containing skirts.

FIG. 13C is a cross-sectional view illustrating the orientation of theskirts and the receiver.

FIG. 14A is an elevation views of a receiver with a plurality ofphotovoltaic-element-containing skirts.

FIG. 14B is a cross-section view illustrating the orientation of theskirts and the receiver, and the interaction of reflected solarradiation with skirt and receiver surfaces.

DETAILED DESCRIPTION

A central receiver system, such as one with a receiver supported on atower, can include at least one solar receiver and a plurality ofheliostats. Each heliostat tracks to reflect light to a target on atower or an aiming point. The heliostats can be arrayed in any suitablemanner. For example, heliostat spacing and positioning can be selectedto provide optimal financial return over a life cycle according topredictive weather data and at least one optimization goal such as totalsolar energy utilization, energy storage, electricity production, orrevenue generation from sales of electricity.

The solar receiver can receive reflected and optionally concentratedsolar radiation and convert the reflected solar radiation to some usefulform of energy, such as heat or electricity. The receiver can be locatedat the top of the receiver tower or at some other location. For example,an intermediate reflector may be used to direct concentrated lightreceived to a receiver at ground level.

Referring now to the figures and in particular to FIG. 1, an example ofa central receiver system 44 is shown. The system 44 can includeheliostats 38 with mirrors 8 that reflect incident solar radiation 28onto a receiver 1. The heliostat-mounted mirrors 8 are capable oftracking the apparent movement of the sun 25 across the sky each day inorder to maintain the reflective focus in the direction of the receiver1 as the angle of the incident radiation 28 changes. The receiver 1 canbe located atop a tower 43. In an alternative example of system 44′,shown in FIG. 2, receiver 1 can be located on the ground, and theheliostat-mounted mirrors 8 reflect solar radiation onto one or moresuspended mirrors 9 which further reflect the radiation onto thereceiver 1.

A fluid (not shown) can be heated in the receiver 1 and conveyed via apipe 47 or other conveyance device (e.g., truck, train, pipeline, etc.)for contemporaneous or later use, for example, to generate power in anelectric power generating plant 45. The heated fluid can also be storedin a minimal heat loss storage unit (not shown) for later use by theelectric power generating plant 45, for example, when solar insolationlevels are below a minimal value. The heat in the fluid can be used inthe generation of electricity by, for example, a turbine employing aRankine, organic Rankine, or Brayton cycle. The fluid may be a workingfluid or intermediate heat transfer fluid (e.g., molten salt) used toheat a working fluid. A thermal storage that includes the heat transferfluid and/or another thermal mass or phase change material may beincluded in the fluid conveyance.

At times, reflected and/or concentrated radiation by theheliostat-mounted mirrors may not be efficiently employed and/orreceived by the receiver. For example, the distribution of reflectedsolar radiation around an intended aiming point on a target surface maybe unpredictable or may be graded (for example, its cross-beam intensitydistribution may be Gaussian). As such, in attempting to achieve atarget uniformity over the surface of the receiver, and in concomitantlydirecting the focus of some heliostats near the receiver edges, somespillage of concentrated light may occur. That is, some of the reflectedradiation aimed near the edges of the boiler misses the receiver.

In the embodiment 101 a of a combined dual mode receiver device shown inFIGS. 3A to 3F, one or more photovoltaic converters 105 are arrangedadjacent the edges of a thermal portion of the receiver 102 a (the term“receiver,” by itself, generally being used herein to refer to thethermal portion of the receiver which may be a boiler, heat exchanger,superheater, or other device used for converting sunlight to heat).Concentrated sunlight that misses the receiver hits the one or morephotovoltaic converters 105. In this way, the so called spillage isconverted into usable electrical energy. The one or more photovoltaicconverters 105 may be an array of photovoltaic cells employing anycrystalline or noncrystalline medium, a thermopile, photochemicalconversion device, or other converter.

Furthermore, the size of the heliostat field in a solar tower system fora given rated electrical output may not be generally determined inaccordance with the maximum expected level of solar radiation but ratherby an optimization of expected financial return projected from thesystem when taking into account the expected distribution of solarradiation over the course of a year as well as other factors which caninclude, for example, differential tariffs. The result of thisoptimization is that there are some hours of peak solar radiation duringthe year in which the total energy available to the solar field exceedsthe rated capacity. As a result of optimizing for financial return,during such peak hours, some heliostats are defocused from the tower toavoid exceeding the rated capacity of the system or one of itscomponents such as a boiler, turbine or transformer, or alternatively toavoid exceeding an output rating mandated by contract or by regulation.This practice is typically referred to as ‘dumping’, and the energy notcaptured by the system as a consequence is called ‘dumped’ energy.

In embodiments, reflected solar radiation that would otherwise be dumpedis captured by an alternate receiver or receiver portion employing asecond energy conversion mode. For example, concentrated sunlight dumpedonto a photovoltaic converter is directly converted to electricity. Sucha photovoltaic converter may be located near or adjacent the thermalreceiver to minimize any required displacement of heliostats for dumpingpurposes. The output derived from the supplemental converters may becaptured and combined with existing systems employed in thethermal-electric generation of power.

The interception of concentrated sunlight resulting from dumping andspillage can be provided by the same energy converter. For example,embodiments in which photovoltaic converters are advantageously locatedadjacent the thermal receiver can have sufficient area to be usable forcapturing concentrated sunlight that is dumped.

For example, a solar power system can include a receiver or receiversection in which a working fluid is heated for conversion in an electricpower generating plant and a supplemental receiver or receiver sectioncapable of efficient photovoltaic conversion of solar radiation toelectricity at concentrations of more than one hundred suns (“highconcentration module”). The two receivers or receiver sections can beintegrated into a common receiver or separated from each other. Thephotovoltaic receiver or receiver section can provide efficientphotovoltaic conversion, for example, at concentrations of more than onehundred suns. The photovoltaic receiver section can incorporatemulti-junction or multi-bandgap photovoltaic cells, a suitable exampleof which is the EMCORE T1000 Triple-Junction High-Efficiency Solar Cellavailable from EMCORE Photovoltaics of Albuquerque, N. Mex.

In another example, a solar power system can include a receiver orreceiver section in which a working fluid is heated for later use in anelectric power generating plant and a receiver or receiver sectioncapable of efficient photovoltaic conversion of solar radiation toelectricity at concentrations of under fifty suns (“low concentrationmodule”). The two receivers or receiver sections can be integrated intoa common receiver or separated from each other. The photovoltaicreceiver or receiver section can provide efficient photovoltaicconversion at concentrations of less than one fifty suns is preferablyone that incorporates single-junction photovoltaic cells made of orbased upon crystalline silicon, a suitable example of which is anArtisun™ silicon cell available from Suniva, Inc., of Atlanta, Ga.

In still another example, a solar power system can include a receiverwith pipes or tubes, or the like, or alternatively a cavity receiver, inwhich a working fluid is heated for later use in an electric powergenerating plant and solar modules for photovoltaic conversion of solarradiation to electricity at concentrations of less than fifty suns(“low-concentration modules”). The low-concentration solar modules caninclude photovoltaic cells made from or based on crystalline silicon.The receiver can also optionally include solar modules for photovoltaicconversion of solar radiation to electricity at concentrations of morethan one hundred suns (“high-concentration modules”). Thehigh-concentration solar modules preferably include multi-junction ormulti-bandgap photovoltaic cells. In some embodiments the working fluidincludes pressurized steam, and in alternative embodiments the workingfluid includes air, carbon dioxide, a metal, or a salt.

The solar receiver section for heating a working fluid can include pipesor tubes, or the like, or alternatively a cavity receiver. The workingfluid can include pressurized steam. Alternatively, the working fluidcan include air, carbon dioxide, a metal or a salt. Examples of suitablereceivers or receiver sections in which a working fluid is heatedinclude any of the receivers described in International ApplicationPublication Nos. WO-2008/154599, filed Jun. 11, 2008, andWO-2009/015388, filed Jul. 28, 2008, the entireties of which are herebyincorporated by reference. Another example is a so-called cavityreceiver in which a fluid in gaseous phase is heated, for example, onedescribed in U.S. Pat. No. 4,633,854, filed Nov. 26, 1985, which is alsoincorporated by reference.

For purposes of clarity, any receiver (or section of a receiver) inwhich a fluid is heated will be called a “boiler” in this specificationeven though in many embodiments the term is used with reference toheating a fluid in a manner not consistent with the usual definition ofthe word boiler. Examples of such embodiments include heating a fluid toa temperature below its boiling point, heating a fluid in a superheateror in a supercritical steam generator, or heating a gaseous fluid in acavity receiver.

In a first mode of operation, heliostats can be aimed so as to focusreflected solar radiation on an external surface of the boiler receiver(in the case of two separate receivers—one a boiler and the otherphotovoltaic—on a tower) or of the boiler portion of a dual-functionreceiver. In this first mode of operation, the presence of thephotovoltaic modules captures at least a portion of the radiation thatwould have become spillage, and converts it efficiently to electricity.The radiation hitting the boiler near the upper and/or lower edges maybe at a concentration of at least one hundred suns and as high as onethousand suns. As such, the solar modules may need to be configured soas to efficiently convert solar radiation at such high concentrations,i.e., include multi-junction or multi-bandgap photovoltaic cells. Thisis particularly true when aiming of the heliostats is done in a way thatseeks to provide a uniform flux distribution over the face of thethermal receiver. Since intensity of each heliostat's beam falls offprogressively from the central axis of the beam, the intensity justbeyond the edge of the thermal receiver may be as high as the intensityjust inside the edge of the thermal receiver. However, some designs maynot provide for critical uniformity in this way. For example, uniformitymay be critical in some systems for superheating receivers, but not forboiling receivers. Or some systems may operate well below the thermallimits of the boiler and be able to tolerate a significant fall off influx from well within the face of the thermal receiver toward the edgesuch that the intensity of spilled concentrated sunlight is much lowerthan the peak. In such cases, it is possible instead to use modulescontaining single-junction photovoltaic cells made of or based uponcrystalline silicon for converting spillage-bound radiation toelectricity.

In a second mode of operation, generally applied during periods of peaksolar radiation, some of the heliostats in a solar power system aredefocused from the boiler (i.e., are not aimed to reflect radiation ontothe boiler) and instead are focused on the photovoltaic receiver orreceiver section. Therefore at least some of the solar energy that wouldotherwise have been dumped is converted to electricity.

As shown in FIGS. 3A-3B, a receiver 101 a can include a star-shapedboiler 102 a in which a fluid conveyed in vertical pipes (not shown) ina boiler 102 a with optional external surfaces 103 a is heated by theincidence of reflected solar radiation on the external surfaces 103 a.The receiver 101 a additionally includes one or more photovoltaicconverters 105, which may incorporate multi-junction photovoltaic cellsor single-junction crystalline silicon cells, depending on the desiredconcentration of reflected sunlight designed to impinge upon themodules. In another example, not illustrated, the one or morephotovoltaic converters 105 can be positioned beneath rather than abovethe fluid-heating section of the receiver. In yet another example of areceiver 101 b illustrated in FIGS. 3C-3D, there can be multiple sets ofone or more photovoltaic converters 105, one positioned above thefluid-heating section 102 b (boiler) and another below. In still anotherexample of a receiver 101 c illustrated in FIGS. 3E-3F, there can be afirst boiler 102 c and a second boiler 102 d, one above the other, withone or more photovoltaic converters 105 interposed between them. Variousshapes for boiler 102 are shown in FIGS. 3A-3F, for example,star-shaped, cylindrical and polygonal. However, the shape of the boileris not limited to the illustrated shapes. For example, the boiler may beoval, elliptical, rectangular or any other shape which optimizes anoperating goal such as maximum conversion of solar energy to electricityor minimum cost per unit of energy. The boiler can optionally have finsor other appendages. Similarly, the shape of the photovoltaic sectionneed not be cylindrical, but can also be star-shaped, rectangular,polygonal, or any other shape which optimizes an operating goal such asmaximum conversion of solar energy to electricity or minimum cost perunit of energy.

The photovoltaic modules may be constructed using any of severaltechnologies. Since some photovoltaic modules have temperatureefficiency curves, their efficiency may be reduced at elevatedtemperatures. Moreover, radiation that is too intense may damage thephotovoltaic modules. Accordingly, the photovoltaic module may beprovided with a fail-safe device for controlling the temperature of thephotovoltaic module in the event of a rapid (undesired or unintentional)increase in flux. In this way, the photovoltaic device can protectitself and provide the heliostat control system time to correct the fluxdistribution leading to the high intensify on the photovoltaicconverter.

In an example of a fail-safe device, the temperature of the photovoltaicmodule is maintained by repositioning the photovoltaic module to changeillumination conditions thereon. FIGS. 3G-3K illustrate a boiler 102with adjacent one or more photovoltaic converters 105 arranged on aplaten 120. The platen 120 may include one or more temperature sensors122. When a temperature threshold is measured by the one or moretemperature sensors 122, a controller 126 can control an actuator 124,which repositions the platen 120 via mechanism 128 to cool or protectthe one or more photovoltaic converters 105. In normal operation, shownin FIG. 3H, the platen 120 may dispose the photocells to receivespillage from the incident radiation 29 on the boiler 102. The platen120 may be rotated, for example, to put the one or more photovoltaicconverters 105 in a position to take advantage of other solar insolationconditions. Such conditions may include a condition where the heliostatsfocus directly on the solar cells (FIG. 3I) or when the solar cellsdirectly receive incident solar radiation (FIG. 3K). When temperatureconditions dictate, the platen 120 may be rotated to place the solarcells in a protected mode (FIG. 3J) which prevents further irradiationof the one or more photovoltaic converters 105 such that its temperaturemay be mitigated.

In another example of temperature control of the one or morephotovoltaic converters 105, a receiver also includes a cooling systemfor removing excess heat from solar modules. As illustrated in FIGS.4A-4B, a receiver includes a boiler 102 and a one or more photovoltaicconverters 105, a cooling pipe 106 in which is conveyed a fluid forremoving heat from the one or more photovoltaic converters 105, and aradiator 107. A controller (not shown) and temperature sensors (notshown) can be used to regulate operation of the radiator to maintain adesired temperature of the one or more photovoltaic converters 105.

Even though photovoltaic cells have the ability to convert sunlightdirectly into electricity, the photovoltaic cells may have limitedefficiency. Thus, at least some of the solar energy directed on thephotovoltaic converters that is not converted to electricity may insteadbe converted to heat energy and discarded to the environment ortemperature control mechanisms. According to a contemplated embodiment,the waste heat of the photovoltaic converters is captured and employedin the heating of the working fluid. As shown in FIG. 4C, a receiverwith a boiler section 102 has one or more photovoltaic converters 105arranged in the same tower. Solar energy incident on the one or morephotovoltaic converters 105 is partly converted to electricity andpartly absorbed as heat in the one or more photovoltaic converters 105.A cooling pipe 132 removes heat from the photovoltaic converters 105 andtransfers it to a heat transfer module 134 in thermal communication withheat transfer or working fluid pipes 136 input to boiler 102. Thus, theheat transfer module 134 serves to preheat the heat transfer or workingfluid before entering the boiler 102.

At other times, focused solar radiation may be unused due to insolationconditions, for example, reflection and occultation losses. In suchscenarios, energy conversion modes can be provided in receivers atalternate focusing locations so as to increase and/or maximizeelectricity generation based on given insolation conditions. Such asolar power generation system can also be advantageously used when aparticular receiver requires dumping, but another receiver may notrequire dumping.

For example, a solar power generation system can include multiple towersand multiple fields of heliostats. The heliostats can be configured tochange their focus from tower to tower in accordance with instructionsreceived from a control system and/or system operator. At least onetower can include a receiver in which a fluid is heated for later use inan electric power generating plant, and at least another tower caninclude a receiver in which solar energy at a concentration of at leastone hundred suns is converted to electricity in photoelectric modules.As previously noted, the photoelectric modules can includemulti-junction or multi-bandgap photovoltaic cells. A controller can beprovided to control the heliostats and their respective aiming points.The controller may alter the aiming points of the heliostats responsiveto at least one of a radiation dumping condition, a cosine losscondition, a heliostat occultation condition, a receiver uniformitycondition, a solar insolation condition, an electricity productionoptimization condition, and a revenue optimization condition.

Thus, heliostats can be defocused from a tower with a boiler and aimedat a tower with photovoltaic converters in order to capture energy thatwould have been otherwise dumped. Additionally, heliostats can bedefocused from a boiler in a first tower and aimed at one or morephotovoltaic converters in a second tower in order to utilize incidentsolar radiation more efficiently, in that cosine losses in a particularsituation can be reduced for a first solar angle by servicing one towerand then switching to a different tower for a different solar angle, asillustrated in FIGS. 5A-5B and 6A-6B.

Cosine losses occur because the effective reflection area of a heliostatis reduced by the cosine of one-half of the angle between incidentradiation and reflected radiation. In FIG. 5A heliostat-mounted mirror 8is directed to reflect incident solar radiation 28 on boiler/receiver 1on a first tower 43, resulting in an angle 220 between the incidentradiation 28 and reflected radiation 29. As can be seen in FIG. 5B, ifheliostat-mounted mirror 8 were to be directed by controller 260 toreflect instead onto photovoltaic receiver 201 on a second tower 243,the resulting angle 222 between incident radiation 28 and reflectedradiation 29 would be larger and cosine losses would be accordinglylarger. Therefore, in this situation, switching focus would likely notserve an optimization goal.

FIGS. 6A-6B show the situation of a different solar angle, and in thissituation larger cosine losses are incurred if focus remains onboiler/receiver 1, and smaller cosine losses can be achieved if focus isswitched to photovoltaic (i.e., solar module) receiver 201. It can beseen in FIGS. 6A-6B that with this different solar angle the resultingangle 226 (FIG. 6B) between incident radiation 28 and reflectedradiation 29 would be smaller than angle 224 (FIG. 6A) and cosine losseswould be accordingly smaller. With the smaller cosine losses, a largerproportion of solar energy will be retained by the system and convertedto a usable form, thereby increasing electricity production and revenuegeneration, and therefore switching focus from boiler/receiver 1 tophotovoltaic receiver 201 would serve at least one optimization goalsuch as maximizing electricity production or revenue generation.

Heliostats can also be redirected by controller 260 or an operator froma boiler receiver to a photovoltaic receiver to reduce the effects ofoccultation, i.e., shadowing and blocking. Shadowing occurs at low sunangles when a heliostat casts its shadow on a heliostat located behindit and therefore, not all the incident solar flux reaches the mirror.Blocking occurs when a heliostat in front of another heliostat blocksthe reflected flux on its way to the receiver. Alleviation ofoccultation losses by redirecting mirrors from one receiver to anothercan be part of a periodic and optimally predictive optimization of asystem, field, or field subset, which takes into account not onlyalleviation of occultation losses but also cosine losses, atmosphericattenuation, spillage and at least one system optimization goal.

FIGS. 7A-7B illustrate how switching focus from the boiler receiver to aphotovoltaic receiver can alleviate shadowing. If both heliostat-mountedmirrors 8 and 8 a are focused on boiler/receiver 1 on a first tower 43,as in FIG. 7A, then at certain sun angles mirror 8 will cast a shadow onmirror 8 a, i.e., part of solar radiation 28 a will not reach mirror 8 abecause of mirror 8. An operator and/or computerized control system 260can direct mirror 8 to switch to focus on a photovoltaic receiver 201 ona second tower 243, as shown in FIG. 7B, and thereby avoid or reduceshadowing on mirror 8 a. Such control may also take into account systemoptimization goals and other factors such as distances, atmosphericattenuation, spillage and added cosine losses.

FIGS. 8A-8B illustrate how switching focus from one receiver to anothercan alleviate blocking. If both heliostat-mounted mirrors 8 and 8 a arefocused on boiler/receiver 1 on first tower 43, as in FIG. 8A, then atcertain sun angles mirror 8 will block the reflected solar radiation 29a from mirror 8 a. An operator and/or computerized control system 260can direct mirror 8 a to switch to a photovoltaic receiver 201 on asecond tower 243 as in FIG. 8B and thereby avoid or reduce blocking.Such control may also take into account system optimization goals andother factors such as distances, atmospheric attenuation, spillage andadded cosine losses. This mode of operation can allow for tighterspacing between heliostats since during periods of high shadowing orblocking some of the affected heliostats can be switched to an alternatetower.

In the foregoing discussion of multiple towers, only examples ofswitching from a boiler receiver to a photovoltaic receiver weredescribed and illustrated, but the same principles apply to switchingfrom a photovoltaic receiver to a boiler receiver, from one boilerreceiver to another, from a boiler section in one tower to aphotovoltaic section in another tower, from a photovoltaic section inone tower to a boiler section in another tower, from one photovoltaicreceiver to another, or from one photovoltaic section to another, all inaccordance with the objective of realizing one or more systemoptimization goal such as maximization of revenue, profit, electricitygeneration or system efficiency. Moreover, although only two towers areillustrated in the figures, greater than two towers are also possible,according to one or more contemplated embodiments.

In another example, a solar power tower system includes at least onefluid-heating tower and at least one photovoltaic tower. The system canadditionally include at least one secondary reflector capable ofreflecting radiation reflected by heliostats on one side of a towersubstantially onto the other side, i.e., onto a portion of the receivernot in ‘line of sight’ from those heliostats.

FIGS. 9A-9B illustrate the use of a secondary reflector in a solar powersystem. Referring now to FIG. 9A, radiation 28 from the sun 25 isincident on a group of heliostats 138 equipped with mirrors on one sideof a tower, and also on a second group of heliostats 140 equipped withmirrors on the other side of the tower. The first group 138 to the leftof first tower reflects the radiation 28 onto the boiler/receiver 1 onfirst tower with large cosine losses and therefore low efficiency, dueto the relatively large angle between incident radiation 28 andreflected radiation 29. While these heliostats could be used moreefficiently for reflecting light onto photovoltaic receiver 201 onsecond tower 243, as described elsewhere herein, it often happens thatthe system controller 260 cannot release the heliostats because thereflected insolation on the left side, however weak it may be, is neededfor proper operation of and/or uniformity of illumination on theboiler/receiver 1. At the same time, the second group 140 reflects theradiation 30 b onto the right side of the same boiler/receiver 1 withsmaller cosine losses and therefore higher efficiency, due to therelatively small angle between incident radiation 30 b and reflectedradiation 32 b. In this case it is possible that more reflected energymay be available on the right side of the boiler/receiver 1 than isrequired or desired.

FIG. 9B illustrates an advantage of the present embodiment. Secondaryreflector 260, shown as installed atop a narrow pylon so as to minimizeshadowing and blocking, can help to redistribute the energy availablefrom heliostat group 140 on the right side of the tower and ‘free up’heliostat group 138 on the left side to switch to photovoltaic receiver201 on tower 243 where its energy can be used more efficiently (becauseof the smaller cosine losses). For example, in FIG. 9B heliostat 142 inheliostat group 140 can remain focused on the boiler/receiver 1 whileheliostat 146 can focus on the secondary reflector 260. Incidentinsolation 30 b is reflected by heliostat 146 as first reflectedradiation 34 b onto the secondary reflector 260, which reflectssubstantially all of the first reflected radiation 34 b as secondreflected radiation 36 b onto the left side of the boiler/receiver 1.More energy can impinge on the left side of the boiler/receiver 1 in thesecond reflected radiation 36 b than would accrue from heliostat mirrorsin heliostat group 138 because of the high cosine losses of the latter.Instead, heliostat mirrors in group 138 can focus reflected radiation 32a′ on the photovoltaic receiver 201 on tower 243, and total solar energyavailable to the overall solar tower system is increased by the presenceof the secondary reflector 220. It should be clear to one familiar withthe art of solar power towers that the left and right sides in thesefigures can be construed as east and west, respectively, or vice versa,and that the low sun angle is typical of the first and last hours ofdaily sunlight. Left and right can also be construed as north and south,with the low sun angle in the south being typical of winter days.Moreover, it should be obvious to one familiar with the art that asecond secondary reflector can be provided so that one is available foran eastern heliostat field in the morning hours, for example, and asecond one available for a western heliostat field in the late afternoonhours.

In another example, it is possible to deploy a secondary reflectorcapable of being moved to various sides of a receiver at various timesof the day or in accordance with seasonal variation in sun position.FIG. 10 shows an example of a secondary reflector 280 with pivoting andmoving capabilities. A secondary reflector 280 is mounted on a wheeledframe 151, which moves freely around the receiver 1 on track 150, inresponse to control instructions. The secondary reflector 280 is alsocapable of pivoting in two axes at the attachment point (not shown) tothe wheeled frame 151.

In various contemplated modes of operation, heliostats can be aimed soas to focus reflected solar radiation directly on a photovoltaicreceiver or receiver section, i.e., on a plurality of photovoltaicconverters. As discussed above, distribution of reflected solarradiation around an intended aiming point on a target surface isapproximately Gaussian. Because of heliostat aiming errors, beam shape,beam divergence and/or other factors, some of the reflected radiationaimed at the photovoltaic receiver misses the photovoltaic convertersand hits a boiler which is positioned above and/or below thephotovoltaic receiver, and where the absorbed radiation heats a fluidconveyed in a pipe, tube or the like. This mode of operation is appliedgenerally in cases where the concentration of reflected radiationdesired for the boiler is less than that desired for the photovoltaicreceiver. An example of such a case is one in which steam is superheatedor reheated in the boiler (or boilers) and a relatively lowconcentration, for example, less than about one hundred suns, is desiredthereupon.

A photovoltaic converter employed in any of the receivers describedherein can be provided with an optical concentrator for additionalconcentration of reflected solar radiation before it reaches the solarmodule. Optical concentrators can include lenses such as Fresnel lenses,and curved mirrors, but any optical element capable of concentratinglight may be used.

Environmental conditions may adversely impact the efficiency of theboiler section of a thermal receiver. For example, prevailing windpatterns interacting with the surface of the boiler may increase heatloss due to convection. Accordingly, fins or ribs can be added to theboiler section to modify the airflow patterns around the boiler sectionso as to reduce and/or minimize thermal losses due to convection. Suchfins or ribs can extended radially from the boiler section and arepositioned where they can effectively reduce convective heat losses fromthe boiler section by disrupting normal airflow related to prevailingwind patterns. Moreover, the fins or ribs can be provided with one ormore photovoltaic converters to capture reflected and/or concentratedsolar radiation that may otherwise be lost or unused.

For example, a receiver can include a boiler section which includestubes, pipes, or the like, in which a fluid is heated, and additionallyincludes a photovoltaic section with one or more photovoltaic cells madeof or based upon crystalline silicon. The photovoltaic section can beconfigured and disposed so as to produce electricity from reflectedradiation that misses the boiler section, i.e., spillage. The one ormore photovoltaic cells can be arranged on fins or ribs extendedradially from the boiler section, as shown in FIGS. 11A-11B.

The receiver can include a square boiler section 102 and fins 404extending radially from the boiler section 102 at its corners. One ormore photovoltaic converters 105 provided on either of the sides of eachof the fins 404 are positioned so as to receive reflected radiation thatmay miss the boiler section 102. The boiler section 102 can be comprisedof individual pipes or tubes 402. Exterior surfaces of the pipes ortubes 402 may form the exterior surface of the boiler 102, or,alternatively, additional exterior surfaces (not shown) can be providedin thermal contact with pipe or tubes 402 to receive the radiationreflected by the heliostats.

In operation, the photovoltaic converters may regularly receivereflected radiation at a concentration of, for example, less than fiftysuns. Depending on the specifications of the photovoltaic cells, it maybe desirable to limit the reflected radiation to a lower concentration,for example, less than twenty suns. A cooling system (not shown) can beprovided to remove excess heat from solar modules, for example bycirculating a fluid along the back side of each module, i.e., the sidenot facing reflected radiation.

When the boiler section 102 is arranged with prevailing winds, as shownby arrow 406, from the east to the west, fins 404 can serve to decreaseconvective heat losses from turbulent airflow that would otherwise becloser to the boiler section 102 in the area to the east of the boiler102. Fins 404 can also decrease convective heat losses fromboundary-layer turbulence in each of the areas north and south of theboiler 102.

FIG. 12 shows an alternative arrangement for fins 404. In thearrangement of FIG. 12, a first fin 404 a and a second fin 404 b areprovided at each corner. Each fin 404 a, 404 b can be provided with oneor more photovoltaic converters 105, as described above. While the fins404 a, 404 b are shown orthogonal to each other, other angles and/orconfigurations with respect to the boiler 102 are also contemplated.

In another contemplated configuration, projections may be providedextending from the top and/or bottom of the boiler section, so as toform a skirt. Such an example is schematically illustrated in FIGS.13A-13B. As shown in FIG. 13A, skirt 408 can be arranged adjacent to atop edge of receiver 102, while skirt 410 can be provided adjacent to abottom edge of the receiver 102. Each panel 408 a, 410 a of therespective skirt 408, 410 can optionally include one or morephotovoltaic converters 105 disposed on its surface in the direction ofradiation reflected by a field of heliostats. The opposite side of panel408 a can also optionally include one or more photovoltaic converters toreceive incident solar radiation directly from the sun. Thus, radiationreflected from heliostats can be received by one or more photovoltaicconverters 105, while projections 408 a and 410 a can serve to alter theboundary layer adjacent to surfaces of boiler 102, thereby minimizingconvective heat loss due to prevailing wind conditions.

The receiver 102 can have a square cross-section, as illustrated inFIGS. 13A-13B. The receiver 102 can have a width 416, for example, of 4m although other dimensions and cross-sectional shaped are alsocontemplated. The receiver can have a height 414 of 25 m. Each skirt408, 410 can extend a height 412 of 5-10 m above the top or bottom ofthe receiver 102. Appropriate angles for the face of each skirt can bedetermined based on the effect of the skirt on convective heat loss andthe range of angles of radiation directed by the heliostats onto thereceiver. As shown in FIG. 13C, the face of the skirt 408 a can bearranged at an angle 418 with respect to a vertical surface of thereceiver 102 of, for example, 30°. The face of skirt 410 a can bearranged at an angle 420 of, for example, 30°. The above dimensions andangles are for purposes of illustration only, and other dimensions andangles are of course possible according to one or more contemplatedembodiments.

In some cases, a portion of the radiation reflected by the heliostatsonto the surface of the boiler 102 may be reflected from the receiversurface and thus unused for heating the heat transfer or working fluid.To capture this previously unused portion, an L-shaped skirt may beprovided on the top and/or bottom of the receiver. Such a configurationis illustrated in FIGS. 14A-14B.

As shown in FIGS. 14A-14B, top skirt 428 can include a plurality ofangled projections 428 a, which extend up and away from the top edge ofboiler 102. The top skirt 428 also includes a plurality of angledprojections 428 b, which extend down and away from boiler 102. Thebottom skirt 430 can include a plurality of angled projections 430 b,which extend down and away from the bottom edge of boiler 102. Thebottom skirt 430 can also include a plurality of angled projections 430a, which extend down and away from the bottom edge of boiler 102, butless than that of projection 430 b.

Preferably the angle of declination of projections 430 a and 430 b ischosen such that the surface of the projection is parallel to the raysof reflected radiation from the farthest heliostat reflected on receiver102, so as not to interfere with radiation incident on the receiver. Forexample, the angle of declination 442 can be 10°. Similarly, the angleof inclination of projection 428 a and the angle of declination ofprojection 430 b can be chosen based on the angles of reflectedradiation by the heliostats to maximize energy production. For example,the angle of inclination 440 can be 80°. The length of projections 428 aand 430 b can be, for example, 4-6 m. The length of projections 430 aand 428 b can be, for example, 2-4 m. The above dimensions and anglesare for purposes of illustration only, and other dimensions and anglesare of course possible according to one or more contemplatedembodiments.

The projections 428 b can be provided with one or more photovoltaicconverters 105 on the bottom surface thereof. Radiation 452 can bereflected by heliostats onto receiver 102. A portion of this radiation452 may be reflected from the surface of the receiver 102 as radiation454. Thus, one or more photovoltaic converters 105 on projections 428 bcan be arranged to capture the reflected radiation and convert it toelectricity. Projections 428 a can also be provided with one or morephotovoltaic converters 105 on a surface thereof facing the field ofheliostats. Radiation 448 can be reflected by heliostats directly at theprojections 428 a (or radiation 448 can be spillage from radiationdirected at the receiver 102). The opposite sides of projections 428 aand 428 b can optionally be provided with one or more photovoltaicconverters 105. For example, when the opposite side of projection 428 bis provided with one or more photovoltaic converters 105, radiation 450reflected from a surface of projection 428 a can be captured andconverted to electricity. In addition, when the opposite side ofprojection 428 a is provided with one or more photovoltaic converters105, radiation 446 directly from the sun may be captured and convertedto electricity.

The projections 430 a can be provided with one or more photovoltaicconverters 105 on a top surface thereof. Similar to the operation ofprojections 428 b, radiation reflected from a surface of the receiver102 can be captured by one or more photovoltaic converters 105 arrangedon the top surface of projections 428 b. In addition, projections 430 bcan be provided with one or more photovoltaic converters 105 on a topsurface thereof facing the field of heliostats. Thus, radiation can bereflected by the heliostats directly at projections 430 b (or spillagefrom radiation directed at the receiver 102) and converted toelectricity.

In another embodiment, a method for generating electricity includesaiming heliostat-mounted mirrors so as to reflect solar radiationsubstantially onto a boiler or boiler section in which a fluid isheated, and additionally includes converting a portion of the reflectedradiation to electricity by photoelectric modules that includemulti-junction or multi-bandgap photovoltaic cells or that alternativelyinclude single-junction photovoltaic cells made of or based uponcrystalline silicon. In an embodiment the photoelectric modules areabove and/or below the boiler. In still another embodiment two boilersare provided, with one below and one above the photoelectric modules. Inyet another embodiment, the reflected radiation incident on thephotoelectric modules is radiation aimed substantially at thefluid-heating receiver or receiver section, and in an alternativeembodiment the reflected radiation incident on the fluid-heatingreceiver or receiver section is radiation aimed substantially at thephotoelectric modules.

In a further embodiment, a method for operating a solar power generationsystem includes aiming at least some heliostat-mounted mirrorssubstantially at a receiver or receiver section which includes tubes,pipes, or the like, in which a fluid is heated, or alternatively acavity in which a gaseous phase fluid is heated, and aiming at leastsome heliostat-mounted mirrors substantially at a plurality ofphotoelectric modules which include multi-junction or multi-bandgapphotovoltaic cells, or which alternatively include single-junctionphotovoltaic cells made of or based upon crystalline silicon.

In yet another embodiment, a method for generating electricity in asolar power generation system includes using part of the solar radiationincident on a plurality of heliostat-mounted mirrors for photovoltaicconversion of electricity in photoelectric modules that includemulti-junction or multi-bandgap photovoltaic cells or that alternativelyinclude single-junction photovoltaic cells made of or based uponcrystalline silicon, and part for thermosolar power generation (i.e.,heating a fluid for use in an electric power generating plant). In apreferred aspect the total electric power generated through thethermosolar and the photovoltaic conversion is greater than the ratedthermosolar generation capacity of the system.

It is, thus, apparent that there is provided, in accordance with thepresent disclosure, solar power generation systems, methods and deviceswith multiple energy conversion modes. Many alternatives, modifications,and variations are enabled by the present disclosure. Features of thedisclosed embodiments can be combined, rearranged, omitted, etc., withinthe scope of the invention to produce additional embodiments.Furthermore, certain features may sometimes be used to advantage withouta corresponding use of other features. Accordingly, Applicants intend toembrace all such alternatives, modifications, equivalents, andvariations that are within the spirit and scope of the presentinvention.

1. A solar energy control method, comprising: controlling a plurality ofheliostats to track the apparent movement of the sun to concentratesunlight on a receiver including first and second adjacent receivingsurfaces, the first surface being configured to convert concentratedsunlight incident thereon into heat and a second surface having aphotovoltaic converter; controlling the plurality of heliostats to aimreflected light at the first receiving surface by arranging image spotsthereon at edges adjacent an interface between the first and secondsurfaces such that sunlight that spills over the edge is substantiallyincident upon the second receiving surface; controlling at least one ofthe plurality of heliostats such that its reflected light is moved fromthe first surface to the second surface responsively to a higherinsolation level and controlling at least one of the plurality ofheliostats such that its reflected light is moved from the secondsurface to the first surface responsively to a lower insolation level,such that a total incident sunlight on the first surface is limitedbelow a total of which the plurality of heliostats is capable, andenergy otherwise lost by spillage at edges of the first surface due tothe aiming of heliostats at the edges is captured by the second surfaceand converted to electrical power by the photovoltaic converter.
 2. Thecontrol method of claim 1, further comprising iteratively controllingflux distribution by aiming the plurality of heliostats multiple timesin a day to different positions on the first surface such that limits ontemperature gradient and maximum temperature are maintained below apotential capability of the plurality of heliostats.
 3. A solar energyconversion system, comprising: at least one heliostat; a receiver withfirst and second surface portions, the first surface portion including aheat exchanger configured to heat a fluid, the heat exchanger beingconnected to a thermal power plant configured to convert heat in thefluid to electric power, the second surface portion including aphotovoltaic surface configured to convert incident light toelectricity; and a controller configured to aim the at least oneheliostat so as to reflect solar radiation onto both the first andsecond portions responsively to a determination of at least one of: auniformity of flux across the first portion; and a total flux on thefirst portion exceeding a predetermined level.
 4. The system of claim 3,wherein the determination includes a uniformity of flux across the firstportion.
 5. The system of claim 3, wherein the determination includes atotal flux on the first portion exceeding a predetermined level.
 6. Thesystem of claim 3, wherein the first and second surface portions aremutually adjacent and supported on a same tower.
 7. The system of claim3, wherein at least a substantial part of the first surface portion issubstantially vertical and at least a substantial part of the secondsurface portion forms an obtuse angle with respect to the at least asubstantial part of the first surface portion.
 8. A solar energyconversion system comprising: a receiver in a first elevated tower; afield of heliostats, each heliostat being configured to track the sun toreflect incident solar radiation at the receiver so as to heat a workingfluid flowing through the receiver; a conveyance device configured totransport heated working fluid from the receiver to an electric powergenerating plant, the heated working fluid being used by the electricpower generating plant in the generation of electricity; and aphotovoltaic converting device arranged adjacent to the receiver so asto receive a portion of the solar radiation reflected toward edges ofthe receiver that misses the receiver.
 9. The system of claim 8, whereinthe photovoltaic converting device is arranged in the first elevatedtower adjacent to the receiver so as to receive spillage of thereflected solar radiation from the receiver.
 10. The system of claim 8,wherein a further photovoltaic converting device is arranged betweenadjacent sections of the receiver.
 11. The system of claim 8, whereinthe photovoltaic converting device is supported on a projectionextending from the receiver, and at least a portion of the photovoltaicconverting device overhangs the receiver so that solar radiationreflected by the heliostats at the receiver and subsequently reflectedby a surface of the receiver is captured by the at least a portion. 12.A solar energy conversion system comprising: a receiver in a firstelevated tower; a photovoltaic conversion device arranged in a secondelevated tower; a field of heliostats, at least some of the heliostatsbeing configured to reflect solar radiation alternatively at thereceiver and at the photovoltaic conversion device, the receiver beingconfigured to use the reflected solar radiation incident thereon to heata working fluid flowing through the receiver; a conveyance deviceconfigured to transport heated working fluid from the receiver to anelectric power generating plant, the heated working fluid being used bythe electric power generating plant in the generation of electricity;and a controller configured to control at least one of the heliostats toreflect solar radiation onto the receiver and at least another of theheliostats to reflect solar radiation onto the photovoltaic conversiondevice.
 13. The system of claim 12, wherein the controller is configuredto control the heliostats to change respective aiming points thereofbetween a surface of the receiver and a surface of the photovoltaicconverting device.
 14. A multi-mode solar energy conversion systemcomprising: a plurality of focusing elements configured to track the sunand to focus incident solar radiation; a receiver configured andarranged to receive focused solar radiation from the focusing elementsso as to heat a fluid; a thermal electric power plant configured toreceive the heated fluid and generate electricity therefrom; and anenergy conversion device configured to generate electricity from atleast a portion of the focused solar radiation not incident upon thereceiver.
 15. The system of claim 14, wherein the focusing elements areheliostats controlled to concentrate energy on the receiver.
 16. Thesystem of claim 14, wherein the energy conversion device includes aphotovoltaic converter.
 17. The system of claim 14, wherein the energyconversion device is arranged to receive spillage of radiation from thereceiver.
 18. The system of claim 14, wherein the energy conversiondevice is positioned and oriented to at least partially face thereceiver such that it receives radiation reflected from a surface of thereceiver.
 19. The system of claim 18, wherein the energy conversiondevice is further positioned adjacent the receiver to receive focusedenergy from the focusing elements that misses the receiver.
 20. Thesystem of claim 14, further comprising: a controller configured torefocus the focusing elements to direct energy from the receiver to theenergy conversion device or from the energy conversion device to thereceiver responsively to at least one of: an available combined quantityof energy from all of the focusing elements exceeding a threshold level,a determination of a reduction in energy incident on the receiver due toshading of at least one of the focusing elements, a determination ofenergy loss due to shading of at least one of the focusing elements byat least another of the focusing elements, and a predicted or identifiedloss of uniformity of flux over a surface of the receiver.
 21. Amulti-mode solar energy conversion method comprising: heating a workingfluid using a first portion of focused solar radiation; generatingelectricity using the heated working fluid; directly generatingelectricity from a second portion of the focused solar radiation; andcontrolling heliostats to reflect incident solar radiation at respectiveaiming points on a surface of a receiver to generate the focused solarradiation, wherein the controlling heliostats includes controlling theheliostats responsively to at least one of: an available quantity offocused radiation from all of the heliostats exceeding a thresholdlevel, a determination of a reduction in energy incident on the receiverdue to shading of at least one of the heliostats, a determination ofenergy loss due to shading of at least one of the heliostats by at leastanother of the heliostats, and a predicted or identified loss ofuniformity of flux over a surface of the receiver.
 22. The method ofclaim 21, wherein the directly generating electricity includesconverting the second portion to electricity using a photovoltaicdevice.
 23. A solar energy receiver for multi-mode solar energyconversion comprising: a first receiver section configured to convey aworking fluid therethrough and to receive focused solar radiationthereon, the first receiver section having a top perimeter edge, abottom perimeter edge, and at least one corner edge extending betweenthe top and bottom perimeter edges; and at least one projectionprojecting from the first receiver section at the top perimeter edge,the bottom perimeter edge, or the at least one corner edge, the at leastone projection being configured and disposed to modify airflow aroundthe first receiver section so as to reduce convection heat loss from thefirst receiver section.
 24. The receiver of claim 23, wherein the atleast one projection modifies boundary layer thickness adjacent to atleast one of the exterior surfaces.
 25. The receiver of claim 23, the atleast one projection being a plurality of projections, each being aplanar extension at a respective corner edge of the first receiversection.
 26. The receiver of claim 23, the at least one projection beinga plurality of projections, each being an angled planar extension at thetop perimeter edge of the first receiver section so as to form a skirtwith a progressively increasing perimeter.
 27. The receiver of claim 23,the at least one projection being a pair of angled planar extensionsforming an L-shaped cross-section and arranged at the top perimeter edgeof the first receiver section.
 28. The receiver of claim 23, theprojection including a photovoltaic cell.
 29. The receiver of claim 23,the at least one projection including at least one photovoltaic cell,the projection being arranged such that the at least one photovoltaiccell captures radiation reflected from the first receiver section oranother projection.
 30. A method of converting solar energy toelectricity, comprising: concentrating sunlight; directing theconcentrated sunlight at a thermal receiver configured to transfer allthe concentrated sunlight received thereby as thermal energy to a heattransfer fluid; and maintaining a temperature uniformity of the thermalreceiver at least in part by periodically redirecting at least a portionof the concentrated sunlight to a photoelectric receiver that convertssaid portion of the concentrated sunlight directly to electricity. 31.The method of claim 30, wherein the directing includes directingconcentrated sunlight at edges of the thermal receiver such that a firstportion of the concentrated sunlight spills over the edges of thethermal receiver, and the maintaining a temperature uniformity includescapturing the first portion of the concentrated sunlight with thephotoelectric receiver, which converts the first portion directly toelectricity.
 32. The method of claim 30, wherein the thermal andphotoelectric receivers are mutually adjacent.
 33. A method ofconverting solar energy to electricity, comprising: directingconcentrated sunlight at a thermal receiver configured to transfer allthe concentrated sunlight received thereby as thermal energy to a heattransfer fluid; maintaining a temperature uniformity of the thermalreceiver at least in part by directing concentrated sunlight at edges ofthe thermal receiver such that a portion thereof spills over the edges;and capturing the spilled portion of concentrated sunlight with aphotoelectric receiver configured to convert energy received therebydirectly to electricity.
 34. The method of claim 33, further comprisingmaintaining a temperature uniformity of the thermal receiver at least inpart by periodically redirecting at least a first portion of theconcentrated sunlight to the photoelectric receiver, which converts thefirst portion directly to electricity.
 35. The method of claim 33,wherein the thermal and photoelectric receivers are mutually adjacent.36. The system of claim 8, wherein the photovoltaic converting device isarranged in said first elevated tower at a different elevation from thereceiver.
 37. The system of claim 14, wherein the focusing elements aredisposed in a region surrounding the receiver in plan view.
 38. Themethod of claim 21, wherein the controlling heliostats includes changingat least one of the respective aiming points of the heliostats.